Monitoring of blood levels of potassium ion concentration is of considerable importance in intensive care medicine. Currently, the measurement is done in a clinical chemistry lab from blood samples drawn at frequent intervals. Although this technique is satisfactory in many situations, very often life-threatening episodes of potassium shift may not be detected by such an intermittent technique. Again, in some patients frequent blood sampling is not desired due to anemic conditions. The need for a continuous real-time monitoring has provided considerable impetus for the development of a new modality for potassium measurement.
Early attempts at designing an in-dwelling sensor for potassium measurement were directed at modifying the potassium electrodes used in the clinical lab machines. However, the possibility of electrical hazard, instability due to the extremely corrosive nature of blood, and need for a reference electrode have seriously hampered progress in this field. Thus, in spite of being relatively older technology, electrodes have found very few applications in in vivo sensors.
More recently, fiber-optic sensors have been developed for in vivo measurement of pH, pCO.sub.2 and pO.sub.2. Important advantages of the optical approach are stability with respect to calibration, absence of electrical hazard and the relative ease of miniaturization. Accordingly, attempts have been made to develop fiber-optic sensors for in vivo measurement of alkali metal ions. Fluorescent probes made by fusing ion selective cryptands with 4-methyl-coumarin dyes are known. However, this scheme is not reversible and therefore not useful for continuous measurements.
Wolfbeis et al, in Analytica Chemica Acta, Vol. 198, pp. 1-12 (1987), have described a probe consisting of rhodamine ester as a fluorophore and valinomycin as an ionophore. Zhujun and Seitz, in SPIE, "Optical Fibers in Medicine III" Vol. 906, pp. 74-79 (1988), have described a similar probe using merocyanine 540 as a fluorophore instead of rhodamine ester. All these schemes appear to be in their experimental stage and have not demonstrated stable sensitivity and amenability to miniaturization. Further, they consist of a Langmuir-Blodgett film, which cannot maintain structural integrity under the conditions of in vivo application. Finally, it is not clear how these schemes could be adopted for fiber optic configurations.
It is clear, however, that there is a need for a potassium sensor for real-time monitoring of potassium ion levels in blood.